1. Field of the Invention
The present invention in general concerns magnetic resonance tomography (MRT) as employed in medicine for examination of patients. The present invention in particular concerns a method and apparatus for high-contrast representation of tissue that exhibits a very short T2 relaxation time (very fast decay of the transverse magnetization Mxy).
2. Description of the Prior Art
MRT is based on the physical phenomenon of nuclear magnetic resonance and has been successfully used as an imaging modality for over 20 years in medicine and biophysics. In this examination modality, the subject is exposed to a strong, constant magnetic field. The nuclear spins of the atoms in the subject, which were previously randomly oriented, are thereby aligned. Radio-frequency energy now can excite these “ordered” nuclear spins to a specific oscillation. In MRT, this oscillation generates the actual measurement signal that is acquired by means of appropriate reception coils. By the use of non-homogeneous magnetic fields generated by gradient coils, the measurement subject can be spatially coded in all three spatial directions, which is generally designated as “spatial coding”.
While the image contrast of an image in x-ray computed tomography (CT) depends only on the electron density of the irradiated tissue, the magnetic resonance signal and therewith the character of the MR image is mainly determined by the intrinsic tissue parameters p, T1 and T2. p is the proton density, T1 characterizes the design of the longitudinal magnetization Mz and T2 characterizes the decay of the transverse magnetization Mxy.
In order to prevent erroneous interpretations of MR images, in the clinical routine a number of MR images with different acquisition parameters are always acquired that are selected such that the image contrast of the individual images is mainly determined by a single tissue parameter. Such images are known as T1-, T2- or p-weighted images. It is also possible to go a step farther and calculate “pure” parameter images from a number of MR images that have been acquired with different acquisition parameters.
The advantage obtained by this technique is that the image contrast in most cases is greater for the calculated parameter images than for the weighted exposures. The MR images thus obtained can ultimately be used in order to characterize various normal and pathological tissue.
Conventionally, it is difficult to show tissue with very short T2 relaxation time with sufficiently positive contrast. Hydrogen nuclei that are integrated into relatively immobile macro-molecular structures normally contribute little image signal, or no image signal at all, because, given very short T2 times (T2<<TE) the magnetic resonance signal of the transverse relaxation at the point in time of the data acquisition already has decayed to zero.
Typical tissues of this type are primarily encountered in the orthopedic field (ligaments, meniscus in the knee) but also, for example, in the parenchyma of the lungs. For example, all conventional spin echo or gradient echo sequences show ligament tears with only very low intensity, such tears are often visible only in front of a background of fatty tissue. If this is lacking, these structures can be very hard to detect.